A number of mutations have been isolated in Drosophila in the structural genes for the enzymes acetylcholinesterase (AChE) and choline acetyltransferase (ChAT). Both conditional and non-conditional mutations exist at each locus. These mutations are being used both independently and in genetic mosaic analyses to study: (1) the biochemistry of AChE and ChAT and (2) the impact of abnormalities in these enzyme activities on the development and physiology of the nervous system and on the behavior of larval and adult individuals.